Nitric acid oxidation and urea modification of carbon fibres as biofilm carriers.

Carbon fibres (CF) are commonly used as carriers in biofilm-based wastewater treatment. The surface properties of the CF are herein modified using a combination of nitric acid oxidation and urea to optimise the carrier to immobilise bacterial cells. The capacity of the CF carriers to immobilise bacterial cells and activated sludge is evaluated using bacterial cell adhesion and sludge immobilisation tests. The total interaction energy profiles between the CF supports and bacterial cells were calculated according to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to explain the mechanism by which these modifications enhance this immobilisation capacity. CF-U has a high capacity for immobilising bacterial cells and activated sludge (3.7 g-sludge/g-CF supports) owing to its low total interaction energy. Nitric acid oxidation reduced the diiodomethane contact angle of CF from 55.1° to 38.5°, which reduced the Lifshitz-van der Waals interaction energy, while urea modification further increased the zeta potential of CF from 12.8 mV to -0.7 mV, thereby reducing the electrostatic interaction energy. Experiments and DLVO theory both determined that a combination of nitric acid oxidation and urea modification significantly enhanced the ability of CF to immobilise microorganisms.

Optimizing carbon fibre supports for bioreactors by nitric acid oxidation and calcium ion coverage according to extended DLVO theory.

Optimizing supports for microorganisms is required for bioreactors. Carbon fibres (CF) were employed as supports for microorganisms. To optimize CF supports for immobilizing bacterial cells, we used methods of nitric acid oxidation and calcium ion coverage. We evaluated the capacity of these CF supports (untreated CF, nitric acid oxidation CF and Ca2+-covered CF) via bacterial cell adhesion tests, based on extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. The results implied that because of the high hamaker constants, oxidized CF supports had higher capacity in this regard than untreated CF supports. However, the growing oxygen groups increased the negative zeta potential of CF supports, thus likely to reduce their capacity, in accordance with XDLVO theory. Since the Ca2+ coverage could decrease the negative zeta potentials of CF without reducing the hamaker constants, it could enhance the capacity of oxidized CF supports. We concluded that a combination of nitric acid oxidation and Ca2+ coverage could increase the capacity of CF supports to immobilize bacterial cells.

Carbon nanotubes/carbon fiber hybrid material: a super support material for sludge biofilms.

Carbon fiber (CF) is widely used as a sludge biofilm support material for wastewater treatment. Carbon nanotubes/carbon fiber (CNTs/CF) hybrid material was prepared by ultrasonically assisted electrophoretic deposition (EPD). CF supports (CF without handling, CF oxidized by nitric acid, CNTs/CF hybrid material) were evaluated by sludge immobilization tests, bacterial cell adsorption tests and Derjaguin -Landau -Verwey -Overbeek (DLVO) theory. We found that the CNTs/CF hybrid material has a high capacity for adsorbing activated sludge, nitrifying bacterial sludge and pure strains (Escherichia coli and Staphylococcus aureus). CNTs deposited on CF surface easily wound around the curved surface of bacterial cell which resulted in capturing more bacterial cells. DLVO theory indicated the lowest total interaction energy of CNTs/CF hybrid material, which resulted in the highest bacteria cell adsorption velocity. Experiments and DLVO theory results proved that CNTs/CF hybrid material is a super support material for sludge biofilms.